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Sydney's earliest fortifications were built around Sydney Cove to protect the new colony of Sydney from attacks from non-British ships or convict uprisings. The first gun emplacement was built in 1801 on Middle Head[2] (north of Obelisk Bay) during the Napoleonic wars. More permanent fortifications were built during the first half of the 19th century in response to feared foreign attacks[3] and new threats to the British empire. In 1790 the Dawes Point Battery was meant to be the first line of defence against an attack by the Spanish Empire, Napoleon’s French troops in 1810, and the Russian Pacific Fleet in the 1850s (during the Crimean War). This was part of a line of defences that were concentrated on protecting the inner Harbour, these also appear to have been given a low priority. Construction of Fort Denison began in 1841, though the fort was not completed until 1857.[4][5][6]

The Beehive Casemate was carved into the cliff face at Obelisk Bay on Sydney Harbour in 1871

A Submarine Miners' Depot was constructed at Chowder Bay (Georges Head) in the 1880s. In 1888 the site was modified for the latest in harbour defences. The site was a strategic position and considered the best place to observe and fire mines which were laid underwater. Minefields were laid across the main shipping channels of Port Jackson from 1876 to 1922 and a base was built at Chowder Bay for the submarine miners (Clifton Gardens). From Georges Head, miners watched for ships entering the harbour. Their job was to explode the mine closest to an approaching enemy ship. Each underwater mine was attached to an electric cable that ran up the cliff to the firing post.[7][8]

The work of the submarine miner was secretive, technical and dangerous. During a demonstration in 1891, a crowd of several thousand watched as a terrible accident killed four miners and injured another eight.[9] The Submarine Mining Corps was dismantled in 1922.

Sydney's defences were expanded over the last decades of the 19th century and eventually incorporated a fort on Bare Island to defend the approaches to Botany Bay.

Sydney's defences were not expanded from their pre-Federation condition until the 1920s. During the 1920s a number of well-constructed fortifications were built to protect Sydney. Sydney's defences were further expanded following the outbreak of World War II and were considerably augmented once Japan entered the war. While Sydney's anti-submarine defences were not yet complete at the time of the attack on Sydney Harbour on the night of 31 May 1942, they were successful in sinking two of the three Japanese midget submarines which attempted to enter the Harbour;[7] one submarine fired two torpedoes at a warship, both of which missed, one hitting another ship.

Following the end of the Japanese threat to Australia Sydney's defences were reduced from 1944. The last fortifications were not decommissioned until the 1960s, however. While Sydney is no longer protected by fixed defences, a high proportion of the Australian Defence Force is based within the greater Sydney region and could be called on to protect the city during an emergency. During the 2000 Sydney Olympics security for the Harbour was provided by Special Forces, Navy Clearance Divers and warships.